A substrate on which plural semiconductor light emitting elements such as light emitting diodes (LEDs) or semiconductor lasers are formed is cut to be divided into semiconductor light emitting chips on each of which a semiconductor light emitting element is mounted (hereinafter, referred to as light emitting chip). As a method for dividing into the light emitting chips, there is a method called a stealth dicing method that focuses laser light by an objective lens optical system, performs irradiation on an interior of a substrate along expected cutting lines assumed on the substrate to form modified regions having crystal strength lower than that before irradiation. In this method, the substrate is cut by starting from the modified regions to be divided into light emitting chips.
In Patent Document 1, there is disclosed a method for cutting a substrate into chips and a device therefor. In the method here, (1) grooves or work damaged layers that become starting points of breaking are formed by a laser, a scribe, a dicer or the like in advance on a substrate to be cut, (2) a blade having an acute-angle tip end is adjusted to contact an opposite surface of the grooves becoming the starting points for breaking, and (3) the blade is pushed into the substrate by application of an impactive force, to thereby cut the substrate by breaking.
Incidentally, if a sapphire single crystal is used as a substrate, though modified regions are formed within the substrate by the stealth dicing method (hereinafter, referred to as laser processing), there was a problem that side surfaces (cutting planes) of the substrate portion in a light emitting chip after being divided were inclined with respect to the surface of the substrate. Hereinbelow, a description will be given by use of FIG. 1.
FIG. 1 is a diagram illustrating crystal orientation of a sapphire single crystal.
The sapphire single crystal belongs to a hexagonal crystal, in which a top surface (bottom surface) of a hexagonal column constituted by an a1-axis, an a2-axis, an a3-axis and a c-axis is a C plane (0001), and all the side surfaces are an M plane (1-100) and planes equivalent to the M plane (M plane {1-100}). Hereinafter, the M plane and the planes equivalent to the M plane are collectively referred to as the M plane. A direction perpendicular to the M plane (1-100) is a [1-100] direction (m-axis). Here, “-” represents a bar above a number following “-”.
A surface that includes a point of a coordinate “1” in each of the a1-axis and the a2-axis and is in parallel with the c-axis is an A plane (11-20).
Five surfaces, each of which includes a line alternately connecting other vertices of a hexagon of the top surface (bottom surface) of the hexagonal column and is in parallel with the c-axis, are surfaces equivalent to the A plane (A plane {11-20}). Hereinafter, the A plane and the planes equivalent to the A plane are collectively referred to as an A plane. A direction perpendicular to the A plane (11-20) is a [11-20] direction (a-axis).
In forming a light emitting chip whose surface shape is rectangular with a scribing method by use of a sapphire substrate, scribing grooves are formed so that one side of the light emitting chip is arranged in parallel with the m-axis direction of the sapphire substrate and another side is arranged in parallel with the a-axis direction of the sapphire substrate.
Next, the sapphire substrate is cleaved along the scribing grooves that are in parallel with the a-axis direction to form plural substrates having a rectangular shape. Then, the rectangular-shaped substrates are cleaved along the scribing grooves that are in parallel with the m-axis direction, to thereby produce light emitting chips each of which has a rectangular surface shape.
In the case where cleavage is performed along the planes that are in parallel with the m-axis direction, since the frequency of inclination of a cutting plane with respect to the surface of the substrate is low, the cutting plane of the substrate is within a cut region that is set to include an expected cutting line, and thereby plural rectangular-shaped substrates can be formed without causing the cutting plane to extend to the semiconductor light emitting element. On the other hand, in the direction in parallel with the a-axis of the above-described LED chip (easy-cleaving plane), the cutting plane is inclined with respect to the surface of the substrate to result in breaking, the cutting plane of the substrate extends off the cut region that is set to include an expected cutting line and extends to the semiconductor light emitting element, and thereby a problem of increasing a reverse current in the semiconductor light emitting element was caused.